The function-based index idea could be very useful here – especially if you use it to exclude the extremely popular values from the index (which, for the sake of argument, I’ll assume is the Male / Female).

An index on something like (case sex when ‘Male’ then null when ‘Female’ then null else sex end) – could be very helpful.

And in some cases your previous thought about multiple function-based indexes (perhaps using 11g’s option for virtual columns and indexes on virtual columns) would be more appropriate with definitions like: (case sex when ‘Unknown’ then ‘Unknown’ else null).

Seriously, if we can not (or we want not) use bitmap indexes, I think that introduction of function based indexes can be of help.